1. Field of the Invention
The present invention relates to a shift control apparatus for an automatic transmission mounted on a vehicle such as an automobile, and more in detail, to a shift control apparatus for an automatic transmission that is capable of providing improvement in a shift shock during a jumping shift by so-called clutch-to-clutch operation (engagement switching).
2. Description of the Related Art
Conventionally, for example, a stepped automatic transmission mounted on a vehicle is capable of shifting speeds by controlling the engagement state of a plurality of friction engagement elements (clutches and brakes) using a hydraulic control device, and thus forming a power transmission path in a speed change gear mechanism at each shift speed. Furthermore, in recent years, multi-speed automatic transmissions have been required to attempt an improvement in vehicle fuel consumption, and in order to select optimal shift speeds corresponding to the driver's requirement (that is, accelerator pedal stroke, and so on). In such automatic transmissions, it has become common to use jumping shifts, that is, speed shifting to a speed spaced apart by two or more steps (for example, 4th-to-2nd, 5th-to-2nd, 2nd-to-4th, and 2nd-to-5th shifts) at a single shifting operation.
Since the range of selection of a shift speed suitable for the driving condition of a vehicle becomes larger in the multiple speed gear train described above, the switching operation between friction engagement elements is not limited to a simple engagement switching using two elements but is required to be a complex switching operation using, for example, four elements. For example, in an engagement switching operation of four elements (so-called dual changeover) in which two friction engagement elements are released and two friction engagement elements are engaged, if, for example, a 6th-to-3rd shift employs a 6th-4th-3rd shift operation using the fourth forward speed as an intermediate shift speed, the torque distribution ratio to a high clutch (C-2) at the intermediate shift speed is smaller than the torque distribution ratio to the high clutch at the sixth forward speed. As a result, the high clutch does not automatically slip during the 4th-to-3rd shift, resulting in a poor controllability of the high clutch. In addition, because the gear ratio changes quickly during the 6th-to-4th shift and the change in gear ratio is suppressed in the narrow range of the 4th-to-3rd shift, the engine is may race due to difficulty in control. Moreover, the clutch distribution ratios of the high clutch on the release side and a 3-5 reverse clutch (C-3) on the engagement side are small during the 6th-to-4th shift which has a large effect on shock, thus causing a variation of hydraulic pressure.
Therefore, in order to address the problems as described above, shift control apparatuses for automatic transmissions have been developed such as described below (refer, for example, to Japanese Patent Application Publication No. JP-A-2003-106440). When the shift control apparatus described in the patent document performs a dual changeover shift in which two friction engagement elements to be engaged and two friction engagement elements to be released are changed over at the same time, shift control is made easy by reducing the time for shifting. Also, hydraulic control during a second changeover shift, which has a large effect on shock, is made easy, thus enabling suppression of shift shocks.
A certain shift control apparatus such as described in the above-mentioned patent document performs feedback control (hereinafter called also FB control) to calculate and output a second shift release pressure required for producing an appropriate rotation change during shifting by engagement switching of pairs of friction engagement elements (engagement switching of four elements). In that case, if the torque capacity of a first shift engagement element to serve as a reaction force element is insufficient, no sufficient effect of the feedback control is obtained, and therefore the control of the rotation change is made difficult, causing a possibility of shift shock generation.